The purpose of this core is to provide support for studies on epithelial Ca 2+ channels (ECaC) and the electrogenic sodium/dicarboxylate co-transporter (NaDC-1) in components I, III, and IV. ECaC channels are present in the apical membrane of intestine and the distal convoluted tubule (DCT) of kidney and serve as gatekeepers for transcellular Ca 2+ transport in these sites. ECaC is a major target for many hormones that regulate gastrointestinal and kidney Ca 2+ absorption, such as parathyroid hormone, vitamin D, etc (1). cDNAs for ECaC and its isoforms have recently been isolated by Dr. Bindels' (2) and Dr. Hediger's groups (3, 4) independently. We have also isolated cDNA for rabbit ECaC1 and obtained cDNA for mouse ECaC2 from Dr. Bindels (consultant for this core) NaDC-1 in the proximal tubules reabsorbs filtered Krebs cycle intermediates and plays an important role in the regulation of urinary citrate concentrations (5). Low urinary citrate is an important risk factor for formation of kidney stone. In component I, we will test the hypothesis that protein product of an absorptive hypercalciuria (AH)-related gene, AH-related adenylate cyclase (AHRAC), regulates ECaC channel activity. In component III project 1, we will investigate the role of intracellular vs extracellular pH in the regulation of the electrogenic NaDC-1 (6). In component III project 2, we will test the hypothesis that acid inhibition of ECaCl-mediated Ca 2+ reabsorption in DCT contributes to the hypercalciuria induced by a high dietary protein intake and the hypothesis that membrane phospholipid, phosphatidylinositol 4,5-bisphosphate (PIP2) regulates ECaC1 channel activity. In component IV, we will test the hypothesis that down-regulation of calbindin-D28k by estrogen lack (in addition to the downregulation of ECaC) is important for renal calcium. leak and examine the molecular mechanism by which calbindin- D28k regulates Ca 2+ reabsorption through ECaC.